Quantum dot based optoelectronic device

ABSTRACT

A device having an optically active region includes a silicon substrate and a SiGe cladding layer epitaxially grown on the silicon substrate. The SiGe cladding layer includes a plurality of arrays of quantum dots separated by at least one SiGe spacing layer, the quantum dots being formed from a compound semiconductor material.

RELATED APPLICATION

This Application is a divisional of and claims priority to U.S. patentapplication Ser. No. 11/169,196 filed on Jun. 27, 2005. Priority isclaimed under 35 U.S.C. §§120 and 121. U.S. patent application Ser. No.11/169,196 is incorporated by reference as if set forth fully herein.

FIELD OF THE INVENTION

The field of the invention generally relates to optoelectronic devicesand methods of making the same. In particular, the field of theinvention relates to optoelectronic devices such as, for example, laserdiodes, light emitting diodes, and photodetectors formed using quantumdots.

BACKGROUND OF THE INVENTION

Optoelectronic devices are becoming increasingly important to a numberof industries such as, for example, the telecommunication industry.Exemplary optoelectronic-based devices include laser diodes (LD), lightemitting diodes (LEDs), and photodetectors (PDs). These devices arefabricated with an optically active region made of semiconductingmaterials that have different lattice constants than the substrate onwhich they are located. Silicon (Si) is a well known substrate materialused in integrated circuit technologies and has developed a maturetechnological base with respect to its use in fabricating integratedcircuits. Unfortunately, silicon is unable to emit light and thereforecannot be used in the “active” portion of optoelectronic devices for theemission or detection of optical radiation.

There have been unsuccessful efforts in the past to integrate compoundsemiconductor materials that are optically active (e.g., they emitoptical radiation) with silicon. The primary obstacle in integratingcompound semiconductor materials in silicon are the crystalline defectsproduced caused by the growth of the compound semiconductor materials onthe silicon substrate. The defects are the result of the relativelylarge lattice mismatch (i.e., different lattice constants) between theadjacent compound semiconductor materials and the underlying siliconsubstrate. For instance, there is an approximately 11% lattice mismatchbetween InAs and Si, and a 4% lattice mismatch between GaAs and Si.InGaAs is an alloy of two compound semiconducting material (InAs andGaAs) that emits light at a wavelength ranging from 0.8 μm to above 1.5μm—the wavelength for most of the optical fiber network that servescurrent telecommunication needs (e.g., the internet and other WANs).

InGaAs, when epitaxially grown on Si <001> substrates, is known to havea critical layer thickness on the order of 10 angstroms. Thus, thethickness of InGaAs which can be grown epitaxially on a Si substrate isbelow 10 angstroms. In comparison, the thickness of a typical quantumwell laser formed from InGaAs is on the order of 2000 angstroms.Consequently, dislocation in InGaAs has been unavoidable. Dislocationintroduced by epitaxial film relaxation severely limits the performanceand useful life of optoelectronic devices including, for example,semiconductor lasers.

There thus is a need for a device and method in which compoundsemiconductor materials are employed on silicon substrates. Preferably,the device can be created by epitaxially forming the optically activeregion of optoelectronic devices on a silicon substrate. Preferably, thedevice may be formed with a very small or limited amount of opticallyactive material.

SUMMARY OF THE INVENTION

The present invention is directed to a method of forming anoptoelectronic device having an optically active region on a siliconsubstrate. In one aspect of the invention, the optically active regionis formed on a silicon substrate using an array of epitaxially grownquantum dots from a compound semiconductor material. For example, thecompound semiconductor material may have lattice mismatch with theunderlying Si substrate.

In another aspect of the invention, the quantity or amount of opticallyactive material (e.g., InGaAs), when present in the form of quantumdots, is minimized while still providing good performance. For example,InGaAs quantum dots may be formed, e.g., optical gain, withoutdislocations. This is in contrast with film-based approaches in whichmuch more material is needed to achieve the level of optical activity,as a result, the lattice mismatch between the compound semiconductormaterial and silicon causes a high density of dislocation in layers ofsufficient thickness for optoelectronic applications.

In one aspect of the invention, a method of forming an optically activeregion on a silicon substrate includes the steps of epitaxially growingan optional silicon buffer layer on the silicon substrate andepitaxially growing a cladding layer having a plurality of arrays ofquantum dots disposed therein, the quantum dots being formed from acompound semiconductor material having a lattice mismatch with thesilicon buffer layer. The optically active region may be incorporatedinto devices such as light emitting diodes, laser diodes, andphotodetectors.

In another aspect of the invention, a device having an optically activeregion includes a silicon substrate and a SiGe cladding layerepitaxially grown on the silicon substrate, the SiGe cladding layercomprising a plurality of arrays of InGaAs quantum dots separated by atleast one SiGe spacing layer.

In another aspect of the invention, a method of forming an opticallyactive region on a silicon substrate includes the steps of epitaxiallygrowing an SiGe etch-stop layer on a silicon substrate and epitaxiallygrowing a silicon buffer layer on the SiGe etch-stop layer. A claddinglayer of SiGe is then epitaxially grown having an array of InGaAsquantum dots epitaxially grown therein. The back side of the siliconsubstrate is then etched followed by an etching of the SiGe etch-stoplayer so as to expose the bottom surface of the silicon buffer layer.The optically active region described above may be interposed betweentwo quarter wave stacks to form a vertical cavity surface emitting laser(VCSEL).

It is an object of the invention to provide a method of fabricatingSi-based optoelectronic devices having optically active regions formedfrom compound semiconductor materials. Devices based on suchhetero-structures (e.g., lattice mismatch) will have commerciallongevity and good performance characteristics. Exemplary products whichmay be produced in accordance with the methods described herein includeSi-based optical transceiver chips, laser diodes, light emitting diodes,and photodetectors.

Further features and advantages will become apparent upon review of thefollowing drawings and description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a schematic representation of an edge emittinglaser.

FIG. 1B illustrates a schematic representation of a surface emittinglaser.

FIG. 2 illustrates a process flow chart of a method of fabricating aSi-based surface emitting laser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A illustrates a schematic representation of an edge emitting laser10. The edge emitting laser 10 includes a p-type silicon substrate 12.An optically active region 14 is formed on the upper surface of thep-type silicon substrate 12. The optically active region 14 includes asilicon (Si) or silicon germanium (SiGe) cladding layer 16 havingdisposed therein one or more layers of an array 18 of quantum dots 20formed from a compound semiconductor material. A quantum dot 20 is acluster of atoms whose dimensions are less than the quantum mechanicalwavelength of an electron or “hole.” In one preferred aspect of theinvention, the quantum dots 20 are formed from semiconductor materialswith a direct energy band gap. Illustrative semiconductor materials forthe quantum dots 20 include InGaAs, InGaSb, PbTe, CdTe, and CdSe. In oneaspect, the quantum dots 20 are formed from a compound semiconductormaterial having a lattice mismatch with the underlying silicon bufferlayer (for example a lattice mismatch of at least 5%). As seen in FIG.1A, the edges of the optically active region 14 may include etchedfacets 15.

In one aspect of the invention, the array 18 of individual quantum dots20 is formed by growing islands or dots of semiconductor materials witha direct energy band gap on an array of preferential nucleation sites.These nucleation sites may be formed, for example, by introducing aseries of perpendicularly-oriented dislocation lines (not shown) byepitaxially growing a strained film such as SiGe and allowing it tosubsequently relax via dislocation. For example, U.S. Pat. No. 5,888,885discloses a method of fabricating three-dimensional quantum dot arrays.The '885 patent is incorporated by reference as if set forth fullyherein. A spacing layer 22 such as a layer formed from SiGe may beinterposed between adjacent quantum dot arrays 18.

Still referring to FIG. 1A, in the case of an edge emitting laser 10,the cladding layer 16 includes an n-type silicon layer 24 disposedthereon. The n-type silicon layer 24 may also be heavily doped withn-type impurity to facilitate an Ohmic contact. The advantage of theoptically active region 14 disclosed in FIG. 1A includes that fact thata very small amount of material (e.g., InGaAs) is sufficient for thedesired optoelectronic functionality when the material is in the form ofquantum dots 20. The small amount of the material under strain in theoptically active region 14 reduces or mitigates the potential forcrystalline defects. A second benefit of the optically active region 14is that the dislocation network permits the formation of more organizedand uniform quantum dots 20. For example, the nanometer-size patterningutilizing the strain field from a buried dislocation network producesmore uniform size distribution of quantum dots 20. This is particularlybeneficial in the case of LDs in which a single wavelength is selectedby the resonant cavity. A tighter size distribution of quantum dots 20results in more quantum dots 20 that participate in the lasingaction—thereby reducing the amount of optically active compoundsemiconductors needed.

FIG. 1B illustrates an embodiment of a surface emitting laser 30. Thesurface emitting laser 30 may be, for example, a silicon-based verticalcavity surface emitting laser (VCSEL) such as that shown in FIG. 1B. Thesurface emitting layer 30 includes a p-type silicon substrate 32 havingthe bottom surface etched to form a cavity 34 that contains a quarterwave stack 36 (QWS). The quarter wave stack 36 is a dielectric coatingformed by depositing alternating layers of high-low index materials,e.g., SiO₂ and TiO₂, in quarter-wave thicknesses. The quarter wave stack36 is able to efficiently reflect optical radiation generated by thequantum dots (described below) during the lasing operation. Theunderside of the p-type silicon substrate 32 includes metal contacts 38.

Still referring to FIG. 1B, a doped, p-type silicon buffer layer 40 isformed on the silicon substrate 32 and interposed between the quarterwave stack 36. An optically active region 42 is formed on the uppersurface of the p-type silicon buffer layer 40. The optically activeregion 42 includes a silicon or silicon germanium (SiGe) cladding layer44 having disposed therein one or more array(s) 46 of quantum dots 48formed from a compound semiconductor. In one aspect of the invention,the quantum dots 20 are formed from a compound semiconductor materialhaving a lattice mismatch with the underlying silicon buffer layer.Alternatively, the quantum dots 20 may be formed by patterning such thatlattice mismatch is not necessary. For example, the quantum dots 48 maybe formed from InGaAs. A spacing layer 50 such as a layer formed fromSiGe may be interposed between adjacent quantum dot arrays 46. A layerof doped n-type silicon is formed to create a top contact layer 52. Asecond quarter wave stack 54 is deposited on the layer 52 by, forexample, sputtering or evaporation.

Oxide isolation regions 56 (SiO₂) are formed adjacent to the opticallyactive region 42. The surface emitting laser 30 includes a top metalcontact layer 58 formed, for example, by evaporating metal over apatterned resist and subsequent removal by a wet-solvent lift-offprocess. An optically transparent layer 60 may be disposed above thequarter wave stack 36.

The direction in which radiation is emitted from the surface emittinglaser 30 may be controlled by adjusting the respective reflectivity ofthe quarter wave stacks 36, 54.

FIG. 2 illustrates a process flow chart for creating a surface emittinglaser 30 of the type illustrated in FIG. 1B. Step 100 involves thecleaning and/or washing of p-type silicon substrate 32. For example,this may include the wet chemical cleaning of the surface of a heavilydoped p-type silicon substrate 32. In step 105, a p-type SiGe etch-stoplayer (not shown), the silicon buffer layer 40, and optically activeregion 42 are epitaxially grown. The silicon buffer layer 40 is grown ontop of the p-type SiGe etch-stop layer using a heavily doped p-typesilicon buffer layer 40 having a thickness of around 3000 angstroms.With respect to the active region 42, an undoped SiGe (with 5% Ge)cladding layer 44 is formed having a plurality of InGaAs quantum dotarrays 46 (e.g., three). As seen in FIG. 1B, the cladding layer 44 mayinclude a SiGe spacing layer 50 for a total thickness of 1000 angstroms.The active region 42 may be topped off with an undoped silicon cap layerhaving a thickness of about 200 angstroms.

In step 110, the laser area of the optically active region 42 is formedor otherwise defined using photolithography. In step 115, oxideisolation regions 56 are formed in the silicon buffer layer 40. Forexample, porous silicon may be formed using conventional techniques suchas, for example, electrochemical etching of the silicon buffer layer 40.The formation of porous silicon preferably stops inside the siliconbuffer layer 40. The silicon substrate 32 and structures formed thereonthen undergoes thermal oxidation to form the oxide isolation regions 56.The resulting oxide isolation regions may have a thickness around 100angstroms.

In step 120 a top contact layer 52 is formed by the non-selectiveepitaxial growth of heavily doped n-type silicon to a thickness of lessthan 1000 angstroms. In step 125, the first quarter wave stack 36 isdeposited, for example, by sputtering or evaporation. The top mirror ofthe first quarter wave 36 is then defined using conventionalphotolithography techniques and the first quarter wave stack 36 issubject to an etching process that forms the QWS 36 and stops in then-type silicon top contact layer 52.

In step 130, the top metal contact layer 58 is then formed, for example,by evaporating or sputtering metal over a patterned resist and thensubsequent partial removal by a wet-solvent lift-off process. Next, instep 135, a layer of silicon oxide is formed on the bottom side of thesilicon substrate 32. The layer of silicon oxide forms an oxide mask forsubsequent etching of the silicon substrate 32.

In step 140, the top side of the silicon substrate 32 (and associatedconstituents) is protected using, for instance, wax or deposited silicondioxide. In step 145, the underside of the silicon substrate 32 is thenetched in, for example, KOH solution. Preferably, the etching is stoppeda few micrometers from the SiGe etch-stop layer grown in step 105.

In step 150, the SiGe etch-stop layer is etched to expose the siliconlayer on top. For example, alternating solutions selective to Si and Gemay be employed to remove the etch-stop layer. In step 155, the secondquarter wave stack 54 is formed by sputtering or evaporation on theunderside of the silicon substrate 32. The second quarter wave stack 54is defined by photolithography followed by reactive ion etching (RIE).In step 160 the bottom silicon oxide mask formed in step 135 is removedand a bottom metal contact layer 38 is formed. The bottom metal contactlayer 38 may be formed by first sputtering or evaporating metal over apatterned resist and subsequent removal by a wet-solvent lift-offprocess.

While FIG. 1B and the process illustrated in FIG. 2 is shown in thecontext of a surface emitting laser 30 it should be understood that thesame or similar structure may be used for photodetectors (PDs). In oneaspect of the invention, the spectral response of the light emittersand/or PDs may be adjusted by controlling the size the quantum dots (20,48).

The methods and devices described herein may be useful for creatingoptically active regions in a variety of devices. For example, opticallyactive regions may be placed into LDs or LEDs integrated withsilicon-based driver circuitry and photodetectors to form integratedtransceivers for high bandwidth fiber optical communications.

While embodiments of the present invention have been shown anddescribed, various modifications may be made without departing from thescope of the present invention. The invention, therefore, should not belimited, except to the following claims, and their equivalents.

1. A device having an optically active region comprising: a siliconsubstrate; and a SiGe cladding layer epitaxially grown on the siliconsubstrate, the SiGe cladding layer comprising a plurality of arrays ofquantum dots separated by at least one SiGe spacing layer, the quantumdots being formed from a compound semiconductor material.
 2. The deviceof claim 1, wherein the optically active region is formed in a laserdiode.
 3. The device of claim 1, wherein the optically active region isformed in a light emitting diode
 4. The device of claim 1, wherein theoptically active region is formed in a photodetector.
 5. The device ofclaim 1, wherein the optically active region is formed in a verticalcavity surface emitting laser.
 6. The device of claim 1, wherein thequantum dots are formed from a compound semiconductor material selectedfrom the group consisting of InGaAs, InGaP, InGaSb, CdTe, CdSe, andPbTe.
 7. The device of claim 1, wherein the quantum dots are formed froma compound semiconductor material having a lattice mismatch with theSiGe cladding layer of at least 4%.
 8. The device of claim 1, furthercomprising an n-type silicon layer disposed on the cladding layer. 9.The device of claim 1, wherein the quantum dots are formed at nucleationsites disposed on perpendicularly-oriented dislocation lines.
 10. Thedevice of claim 1, wherein the optically active region comprises etchedfacets.
 11. A device having an optically active region comprising: asilicon substrate; and a cladding layer epitaxially grown on the siliconsubstrate, the cladding layer comprising a plurality of stacked arraysof quantum dots, the quantum dots being formed from a compoundsemiconductor material; wherein the quantum dots are formed from acompound semiconductor material having a lattice mismatch with thecladding layer of at least 4%.
 12. The device of claim 11, wherein theoptically active region is formed in a laser diode.
 13. The device ofclaim 11, wherein the optically active region is formed in a lightemitting diode
 14. The device of claim 11, wherein the optically activeregion is formed in a photodetector.
 15. The device of claim 11, whereinthe optically active region is formed in a vertical cavity surfaceemitting laser.
 16. The device of claim 11, wherein the quantum dots areformed from a compound semiconductor material selected from the groupconsisting of InGaAs, InGaP, InGaSb, CdTe, CdSe, and PbTe.
 17. Thedevice of claim 11, further comprising an n-type silicon layer disposedon the cladding layer.
 18. The device of claim 11, wherein the quantumdots are formed at nucleation sites disposed on perpendicularly-orienteddislocation lines.